Instrumentation
Classification of Instruments :
1. Active / Passive Instruments
Active : Here , transducer itself is the source of power.
Eg : Displacement sensor ( Potentiometer ) , Liquid level indicator. ,
Thermocouple.
Passive : It does itself generate any electrical power , i.e it requires
external source.
Eg : Pressure sensor , glass thermometer , Bourdon tubes .
Instrumentation
2. Null / Deflection type :
Null type : Here , a null / zero indication will lead to the determination
of the magnitude of measured quantity .
They have high accuracy and sensitivity.
eg : dc potentiometer , Wheat stone's bridge.
Deflection type : Here , deflection of the instrument forms basis of
measurement.
They are more suitable for measurements under dynamic conditions
eg : PMMC ammeter , Wheat stone's bridge.
Instrumentation
3. Monitoring / Transmitting Instruments :
Monitoring type :
eg : Flowmeter , LVDT , strain gauges.
Transmitting type :
eg : RTD , Thermocouple.
Some Instruments can be both Monitoring and Transmitting type.
4. Analog / Digital Instruments :
Instrumentation
Example of a typical Process Instrumentation system :
Ultimate goal : Rate of flow of liquid suppose L liters / min must be
maintained constant
D/P Transmitter : According to flow of liquid , it gives pneumatic
signal i.e 3 – 15 PSI ( pounds per square inch ) .
If flow increases , error signal decreases and controller takes action by
slowly closing the valve , thereby reducing the flow rate until it
reaches the set point as required.
Similar action takes place when flow rate decreases .
Instrumentation
Basic Characteristics of instruments / Performance
Characteristics of measuring Systems :
Knowledge of basic characteristics of instruments is important for
selecting the most suitable instrument for specific measuring purposes.
It can be divided into two categories :
1. Static Characteristics
2. Dynamic Characteristics
Instrumentation
Static Characteristics :
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Static Characteristics of an instrument are concerned only with steady
state readings.
Span
If in a measuring instrument the highest point of calibration is x2 units
and the lowest point of calibration is x1 units.
Then the instrument range is x2 units.
The instrument span is given by :
Span = ( x2 – x1 ) units
Instrumentation
Accuracy
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Accuracy is usually expressed as ' accurate to within x % '.
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It means ' accurate to within ± x % of instrument span / reading '.
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When a temperature sensor with an error of ± 1 % indicates 100º C ,
the true temperature is somewhere between 99 and 100º C .
Thus the measured accuracy of ± 1 % defines how close the
measurement is to actual measured quantity .
Instrumentation
Precision
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Precision is a measure of the degree to which successive
measurements differ from each other.
Linearity
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If the calibration curve of an instrument is not a straight line then it is
not a linear instrument however it may be very accurate.
It is normally desirable that the output reading of an instrument is
linearly proportional to the quantity being measured.
Non-linearity if defined as maximum deviation of any of the output
readings marked with a crossed from this straight line and it is
expressed as a % of full scale reading.
Instrumentation
Tolerance
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It is a term that is closely related to accuracy and defines the
maximum error which is to be expected on same value.
It is not exactly a static characteristics, however some instruments
quote as a tolerance figure.
It describes the maximum deviation of a manufactured component
from some specified value.
For example , a resistor having a nominal value of 100 Ω and
tolerance 5 % might have an actual value anywhere between 95 and
105 Ω.
Instrumentation
Static Error
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It is the difference between the true value of a time invariant
measurable quantity and the value indicated by the instrument.
It is the deviation from the true value of the measured variable.
For static error in units : True value + static error = Instrument
reading or True value = Instrument reading – Static error
Instrumentation
Repeatability
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The repeatability of an instrument is the degree of closeness with
which a measured quantity may be repeatedly measured.
Mathematically , it is defined as the measured data known as the
standard deviation σ
Static Sensitivity
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The slope of a static calibration curve , evaluated at the input , yields
the static sensitivity. i.e if calibration curve is linear , then the
sensitivity can be defined as slope of the curve.
Sensitivity is ratio of change in output of instrument to the change in
input.
Instrumentation
Instrumentation
Dead Zone
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Dead zone is the largest value of the measured variable for which the
instrument does not respond .
Dead zone usually occurs with friction in a mechanical measuring
system.
Hysteresis
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Hysteresis error refers to difference between upscale sequence of
calibration and down scale sequence of calibration or
The output at the particular input while increasing and decreasing
varies because of friction or hysteresis damping.
Instrumentation
Resolution
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The measurement resolution of an instrument defines the smallest
change in measured quantity that causes a detectable change in its
output .
It is smallest increment in the input value that can be detected by the
instrument.
For example , in a temperature transducer if 0.2º C is smallest
temperature change that is observed , then the measurement resolution
is 0.2º C.
Instrumentation
Bias
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It means a constant error exists over the full range of measurement of
an instrument.
The error can be easily removed by calibration .
For eg , a voltmeter shows a reading of 1 Volts with no input to its
terminals
If now a known voltage of 30 V is applied to the voltmeter , the
reading would be 31 V.
This constant bias of 1 V can be removed by calibration or by
mechanical / electronic adjustment in the voltmeter itself .
Instrumentation
Drift
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The calibration of an instrument is usually performed under some
controlled conditions of temperature , pressure , etc .
As the variations occur in the ambient temperature etc , static
characteristics change namely zero drift and sensitivity drift .
Zero Drift
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It is the deviation of the output from its zero value , when the
variable to be measured is constant .
The effect of zero drift is to impose bias on the instrument .
Sensitivity Drift
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It defines the amount by which an instrument sensitivity
measurement varies as ambient conditions change .
Instrumentation
Calibration
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It is a procedure that involves a comparison of particular instrument
with either (1) a primary standard (2) a secondary standard with a
higher accuracy than the instrument to be calibrated (3) a know output
source .
For example (1) A flow meter can be calibrated with a standard
flowmeter available in standard laboratory.
(2) It can be calibrated with another flowmeter of know accuracy.
(3) It can be directly calibrated with a primary measurement such as
weighting a certain amount of water in a tank and recording time
required for this quantity to flow through the meter.